[Python-Dev] relative import circular problem

[Guido van Rossum]

+1 on Brett and PJE just doing this.

On Thu, Apr 4, 2013 at 3:38 PM, Brett Cannon <brett at python.org> wrote:
>
>
>
> On Thu, Apr 4, 2013 at 5:00 PM, PJ Eby <pje at telecommunity.com> wrote:
>>
>> On Thu, Apr 4, 2013 at 4:42 PM, Guido van Rossum <guido at python.org> wrote:
>> > I do think it would be fine if "from a import b" returned the
>> > attribute 'b' of module 'a' if it exists, and otherwise look for
>> > module 'a.b' in sys.modules.
>>
>> Technically, it already does that -- but inside of __import__, not in
>> the IMPORT_FROM opcode.
>>
>> But then *after* doing that check-and-fallback, __import__ doesn't
>> assign a.b, because it assumes the recursive import it called has
>> already done this...
>
>
> It's an unfortunate side-effect of having loaders set sys.modules for new
> modules not also set them as an attribute on their parent package
> immediately as well (or you could argue it's a side-effect of not passing in
> a module instead of a name to load_module() but that's another discussion).
>
>>
>>  which means that when __import__ returns, the
>> IMPORT_FROM opcode tries and fails to do the getattr.
>>
>> This could be fixed in one of two ways.   Either:
>>
>> 1. Change importlib._bootstrap._handle_fromlist() to set a.b if it
>> successfully imports 'a.b' (inside its duplicate handling for what
>> IMPORT_FROM does), or
>
>
> It's three lines, one of which is 'else:'. Just did it.
>
>>
>> 2. Change the IMPORT_FROM opcode to handle the fallback itself
>>
>>
>> While the latter involves a bit of C coding, it has fewer potential
>> side-effects on the import system as a whole, and simply ensures that
>> if "import" would succeed, then so would "from...import" targeting the
>> same module.
>>
>> (There might be other fixes I haven't thought of, but really, changing
>> IMPORT_FROM to fallback to a sys.modules check is probably by far the
>> least-invasive way to handle it.)
>
>
> This is my preference as well. The change would be small: I think all you
> need to do is if the getattr() fails then fall back to sys.modules. Although
> if it were me and I was casting backwards-compatibility to the wind I would
> rip out the whole fromlist part of __import__() and let the bytecode worry
> about the fromlist, basically making the import opcode call
> importlib.import_module().
>



-- 
--Guido van Rossum (python.org/~guido)